Running out of space on your HD? I am on a few of my machines. I found the following small Perl script that does a great job helping me locate redundant files. It recurses through a given root directory and looks for files of the same size. When it finds two files of the same size, it does an md5sum on them to determine if they're duplicates. It runs pretty quickly and is easy to tweak to pipe into other scripts.
Find duplicate files.